PROJECT SUMMARY & ABSTRACT Mammalian kidney function is critically dependent on the number of nephrons generated during renal development. Nephrons are the filtration unit of the renal system and arise from a nephron progenitor cell (NPC) population at the periphery of the developing tissue. NPCs interact with the surrounding ureteric bud (UB) and stromal compartments, balancing self-renewal and differentiation into segmented nephron structures via a mesenchymal-to-epithelial transition. Consequently, nephron endowment is a quantitative outcome determined by several processes including UB branching and NPC dynamics. Two noteworthy aspects of mammalian renal development are: (1) a 10-fold variation in nephron number (NN) between human kidneys from different individuals, ranging from 200,000 to >2.5 million units per kidney and (2) the synchronous depletion of remaining progenitors at postnatal day 3 in mice (gestational week 34-37 in humans). These facts pose compelling research questions, as the genetic contributions to these aspects of renal organogenesis are not currently known. From a clinical standpoint, a low nephron endowment, which is particularly prevalent in premature birth cohorts, contributes to high blood pressure and chronic kidney disease (CKD). These conditions pose an immense disease burden worldwide, particularly as there is no known postnatal generation of new nephrons. While various genetic and perinatal factors are demonstrated to reduce NN, there remains a clear need to identify genetic contributions to the variation in and upper limits of nephron endowment. The principal investigator herein has identified that distinct mouse strains can be used to model and dissect the genetic basis of differences in nephron number, as several inbred strains and diversity outbred hybrids exhibit distinct, consistent NN phenotypes. Therefore, this proposal sets forth a strategy to identify and subsequently target genetic loci that modify NN outcomes, leveraging QTL mapping algorithms, sequencing data and known gene expression patterns in renal tissue. Secondarily, on a mechanistic basis, it is unclear whether NN variation arises from altered cessation timing, intrinsic changes in NPC activity, or a combination thereof; cellular energetics and mitochondrial function have been implicated. Thus, this proposal will also investigate a mitochondrial mutant mouse model, which exhibits NN elevated above baseline littermate controls, to identify mechanisms by which nephrogenesis can be enhanced. Collectively, by identifying targets and mechanisms that segregate with either high or low nephron number, this research will contribute to the ability to develop diagnostic screens and interventional treatment strategies for deficient nephrogenesis, respectively. Comprehensively, this research plan will aptly be executed in the fulfillment of a fellowship research training plan aimed at fostering the development of an independent physician-scientist in academic pediatric nephrology.